In modern integrated circuits (ICs), Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are being constructed at sizes close to their physical limits. The scaling of these devices in order to accommodate the ever increasing demand for faster and more complex Integrated Circuits has resulted in increasingly high electric fields within these MOSFETs. These increased electric fields can contribute to reliability problems for the long term operation of these devices.
One such mechanism that causes reliability problem is hot carrier injection (HCI). Hot carrier injection is a phenomenon in solid-state electronic devices where an electron or a “hole” gains sufficient kinetic energy to overcome a potential barrier necessary to break an interface state. The term “hot” refers to the effective temperature used to model carrier velocity, not to the overall temperature of the device. Since the charge carriers can become trapped in the gate dielectric of a MOS transistor, the switching characteristics of the transistor can be permanently changed.
Another mechanism that causes reliability problems seen in integrated circuits results from time-dependent gate dielectric breakdown (or time dependent dielectric breakdown—TDDB). Time dependent dielectric breakdown is a failure mechanism in MOSFETs that occurs when the gate dielectric breaks down as a result of long-time application of relatively low electric field (as opposite to immediate breakdown, which is caused by strong electric field). The breakdown is caused by formation of a conducting path through the gate dielectric to substrate due to electron tunneling current. This typically occurs when MOSFETs are operated close to or beyond their specified operating voltages. Yet another mechanism that can cause reliability problems is bias temperature instability (BTI) (a.k.a., negative bias temperature instability—NBTI).